You could use the date command to get the current time before and after performing the work to be timed and calculate the difference like this:
#!/bin/bash
# Get time as a UNIX timestamp (seconds elapsed since Jan 1, 1970 0:00 UTC)
T="$(date +%s)"
# Do some work here
sleep 2
T="$(($(date +%s)-T))"
echo "Time in seconds: ${T}"
printf "Pretty format: %02d:%02d:%02d:%02d\n" "$((T/86400))" "$((T/3600%24))" "$((T/60%60))" "$((T%60))""
Notes:
$((…)) can be used for basic arithmetic in bash – caution: do not put spaces before a minus – as this might be interpreted as a command-line option.
See also: http://tldp.org/LDP/abs/html/arithexp.html
EDIT:
Additionally, you may want to take a look at sed to search and extract substrings from the output generated by time.
EDIT:
Example for timing with milliseconds (actually nanoseconds but truncated to milliseconds here). Your version of date has to support the %N format and bash should support large numbers.
# UNIX timestamp concatenated with nanoseconds
T="$(date +%s%N)"
# Do some work here
sleep 2
# Time interval in nanoseconds
T="$(($(date +%s%N)-T))"
# Seconds
S="$((T/1000000000))"
# Milliseconds
M="$((T/1000000))"
echo "Time in nanoseconds: ${T}"
printf "Pretty format: %02d:%02d:%02d:%02d.%03d\n" "$((S/86400))" "$((S/3600%24))" "$((S/60%60))" "$((S%60))" "${M}"
DISCLAIMER:
My original version said
M="$((T%1000000000/1000000))"
but this was edited out because it apparently did not work for some people whereas the new version reportedly did. I did not approve of this because I think that you have to use the remainder only but was outvoted.
Choose whatever fits you.